Grease thickened with lithium soap of hydroxy fatty acid and lithium salt of aliphatic dicarboxylic acid

ABSTRACT

A MULTIPURPOSE GREASE HAVING A DROPPING POINT IN EXCESS OF 5000* F. IS PREPARED BY USING AS THE GREASE THICKENER A COMBINATION OF A LITHIUM SOAP OF A C12 TO C24 HYDROXY FATTY ACID, E.G. 12-HYDROXY STEARIC ACID AND A LITHIUM SOAP OF A C2 TO C12 ALIPHATIC DICARBOXYLIC ACID, E.G., AZELAIC ACID, AND BY EMPLOYING A PARTICULAR SEQUENCE OF STEPS THAT INCLUDES THE SEPARATE FORMATION OF THE INDIVIDUAL SOAPS IN THE BASE STOCK OF THE GREASE, THE HYDROXY FATTY ACID SOAP BEING FORMED FIRST.

United States Patent 3,791,973 GREASE THICKENED WITH LITHIUM SOAP OF HYDROXY FATTY ACID AND LITHIUM SALT OF ALIPHATIC DICARBOXYLIC ACID Syed S. H. Gilani, Madison Heights, Mich., and Donald W. Murray, Sarnia, Ontario, Canada, assignors t0 Esso Research and Engineering Company No Drawing. Continuation-impart of abandoned application Ser. No. 118,543, Feb. 24, 1971. This application Feb. 18, 1972, Ser. No. 227,622

Int. Cl. C10m 5/14 U.S. Cl. 252-41 Claims ABSTRACT OF THE DISCLOSURE A multipurpose grease having a dropping point in excess of 500 F. is prepared by using as the grease thickener a combination of a lithium soap of a C to C hydroxy fatty acid, e.g. 12-hydroxy stearic acid and a lithium soap of a C to C aliphatic dicarboxylic acid, e.g., azelaic acid, and by employing a particular sequence of steps that includes the separate formation of the individual soaps in the base stock of the grease, the hydroxy fatty acid soap being formed first.

REFERENCE TO RELATED APPLICATION This is a continuation-in-part of application Ser. No. 118,543 filed Feb. 24, 1971 and now abandoned.

BACKGROUND OF THE INVENTION This invention is concerned with the preparation of a lithium soap lubricating grease having a high dropping point. Lithium soap greases have been known and Widely used for many years. The principal advantages of a lithium soap grease have included high water resistance and ease of dispersion of the soap in all types of lubricating oil base stocks. While the lithium soaps used as thickening agents for these greases can be prepared by reaction of lithium hydroxide or other lithium base with conventional high molecular weight fatty acids, lithium hydroxy stearate and the lithium soaps of related hydroxy fatty acids have been particularly useful because of their great mechanical stability.

There are many fields of application of grease compositions where a high dropping point is required, as for example in the lubrication of traction motor bearings. Such traction motors are used to propel modern diesel locomotives. The engines of the diesel locomotives generate direct current which is then used to run traction motors which are geared directly to the driving axle and wheel assemblies in each truck of the locomotive. A single traction motor may contribute 200 horsepower, and constitute or more of the total motor power of the locomotive. The bearings of these locomotives may be required to operate for periods of as much as three years without any maintenance, and temperatures as high as 250 F. can be reached in such bearings.

DESCRIPTION OF THE INVENTION In accordance with the present invention, a lithium soap grease having a dropping point in excess of 500 F. is prepared from a C to C hydroxy fatty acid and from a C to C dicarboxylic acid using a particular sequence of steps that includes separate formation of the individual soaps.

Although the preparation of lithium soap greases from a mixture of monocarboxylic acids and dicarboxylic acids is known in the art, the present invention provides a method for making greases of 500 F. or more dropping point directly from the carboxylic acids rather than by use of esters of the acids, as taught for example in US. Pat.

2,898,296. U .8. Pat. 2,940,930 also teaches that high dropping point greases can be prepared from mixtures of monocarboxylic and dicarboxylic acids. However, in preparing the greases described in that patent, it was necessary to also include a glycol. The presence of a glycol is undesirable because it renders the grease prone to oxidation and makes the water resistance of the grease undesirably low in some applications. The present invention makes possible the preparation of a grease from a combination of hydroxy fatty acid and dicarboxylic aliphatic acid without the necessity of incorporating a glycol.

The total soap content of the grease of the present invention will, of course, be sufiicient to thicken the composition to grease consistency and will normally be in the range of from about 2 to 30 wt. percent and preferably from about 5 to 20 wt. percent. The proportion of dicarboxylic acid to hydroxy fatty acid will be in the range of about 0.2 to about 1.0, and preferably about 0.5 to 0.8, mole of dicarboxylic acid per mole of hydroxy fatty acid.

The hydroxy fatty acid employed in preparing the greases of this invention will have from about 12 to 24, or more usually about 16 to 20 carbon atoms, and will preferably be a hydroxy stearic acid, e.g., 9-hydroxy, l0- hydroxy, or l2-hydroxy stearic acid, more preferably the latter. Ricinoleic acid, which is an unsaturated form of 12-hydroxy stearic acid, having a double bond in the 9-10 position, can also be used. Other hydroxy fatty acids include 12-hydroxy behenic acid and 10-hydroxy palmitic acid.

The dicarboxylic acid used in the greases of this invention will have from 2 to 12, preferably 4 to 12, and most preferably 6 to 10 carbon atoms. Such acids include oxalic, malonic, succinic, glutaric, adipic, suberic, pimelic, azelaic, dodecanedioic and sebacic acids. Sebacic acid and azelaic acid are particularly preferred.

The lubricating oil base that is used in preparing the grease compositions of this invention can be any of the conventionally used mineral oils, synthetic hydrocarbon oils or synthetic ester oils. In general, these lubricating oils will have a viscosity in the range of about 35 to 200 SUS at 210 F. Mineral lubricating oil base stocks used in preparing the greases can be any conventionally refined base stocks derived from paraflinic, naphthenic and mixed base crudes. Synthetic lubricating oils that can be used include esters of dibasic acids, such as di-Z-ethylhexyl sebacate, esters of glycols such as a C oxo acid diester of tetraethylene glycol, or complex esters such as one formed from 1 mole of sebacic acid and 2 moles of tetraethylene glycol and 2 moles of 2-ethylhexanoic acid. Other synthetic oils that can be used include synthetic hydrocarbons such as alkyl benzenes, e.g., alkylate bottoms from the alkylation of benzene with tetrapropylene, or the copolymers of ethylene and propylene; silicone oils, e.g., ethyl phenyl polysiloxanes, methyl polysiloxanes, etc.; polyglycol oils, e.g., those obtained by condensing butyl alcohol with propylene oxide; carbonate esters, e.g., the product of reacting C oxo alcohol with ethyl carbonate to form a half ester followed by reaction of the latter with tetraethylene glycol, etc. Other suitable synthetic oils include the polyphenyl ethers, e.g.,those having from about 3 to 7 ether linkages and about 4 to 8 phenyl groups. (See US. Pat. 3,424,678, column 3.)

To obtain a grease of very high dropping point in accordance with the present invention, a mixture of lubricating oil and lithium soap of a C to C hydroxy fatty acid is first prepared and then a C to C aliphatic dicarboxylic acid is added to that mixture and converted to its dilithium soap under conditions that will ensure the formationof a complex between the lithium soap of the dicarboxylic acid and the lithium soap of the hydroxy fatty acid. There is evidence to indicate that when the aliphatic dicarboxylic acid is being neutralized with lithium base in the presence of the lithium soap of hydroxy fatty acid, two competing reactions are taking place. In one of these reactions either the dicarboxylic acid or its monolithium soap is being incorporated into the crystal lattice of the lithium soap of hydroxy fatty acid, thereby altering its structure. The second competing reaction is the conversion of the dicarboxylic acid to its dilithium soap. Experimental evidence indicates that it is merely necessary to maintain conditions such that the first reaction occurs more rapidly than the second reaction in order to get the desired complex. The principal factors which control the relative rates of reaction include reaction temperature and the rate at which the lithium base is added to bring about the conversion of the dicarboxylic acid to its dilithium soap. Thus in the case of neutralization of the dicarboxylic acid with an aqueous solution of lithium hydroxide, if the reaction is conducted below about 190 F. the complexing reaction will be relatively slow compared with the neutralization reaction and a high dropping point grease will not be obtained, unless the lithium hydroxide is added very slowly. Above about 215 F. the complexing reaction is much more rapid, and it is virtually impossible to add the lithium hydroxide at a sufficiently rapid rate to interfere with the complexing reaction, particularly when the mixture of lubricating oil and lithium soap of hydroxy fatty acid has been subjected to a dehydration treatment.

While the lithium soap of the hydroxy fatty acid could be preformed and then dispersed in the lubricating oil medium it is generally more expedient to prepare that soap in situ in the lubricating oil by neutralizing the hydroxy fatty acid with lithium base, which will generally be lithium hydroxide. The usual procedure is to charge into the grease kettle from about one-fourth to about onehalf of the total amount of lubricating oil base that will be finally incorporated into the final grease and to then add the hydroxy fatty acid. The mixture of fatty acid and oil is heated sufficiently to bring about the dissolving action, e.g., at about 180 to 200 F. Then a concentrated aqueous solution of lithium hydroxide is added, usually in an amount slightly in excess of that required to neutralize the acid, the temperature at this stage being usually about 200 to 210 F. The rate of addition of the lithium hydroxide at this stage is not critical although, depending on the facilities at the grease plant, this may require from 30 minutes to about 2 hours. It is possible at this stage to proceed with the addition of the dicarboxylic acid and its subsequent neutralization to its dilithium soap but this would require the neutralization to be conducted very slowly or stepwise so as to ensure complexing of the two types of soaps with each other before the complete neutralization of the dicarboxylic acid has been brought about. Accordingly, it is preferred to raise the temperature of the mixture of lubricating oil and lithium soap of hydroxy fatty acid to a range of about 250 to 300 F. to bring about a substantial dehydration of the mixture, i.e., the removal of 70 to 100% of the water, before proceeding with the addition of dicarboxylic acid and conversion to its dilithium soap. It has been found that substantial dehydration at this stage also promotes the subsequent complexing reaction during the neutralization of the dicarboxylic acid. After substantial dehydration has been brought about, the mixture is cooled to about 230 to 240 F. as rapidly as possible, the speed of cooling being primarily to save time. Then the dicarboxylic acid is added, the mixture is stirred for a short time, e.g., minutes to bring about proper dispersion throughout the mixture, and then a concentrated aqueous solution of lithium hydroxide is added to convert the dicarboxylic acid to its dilithium soap. Normally, the amount of lithium hydroxide added at this stage is slightly in excess of the amount theoretically required to neutralize both acid groups of the dicarboxylic acid. During the addition of the lithium hydroxide, which will ordinarily take from about 30 minutes to 2 hours, the temperature will be held within the range of about 210 to 230, preferably about 220 to 230 F. Because of the cooling effect brought about by vaporization of water from the aqueous solution of lithium hydroxide as well as water produced in the reaction, maintenance of a temperature of about 220 to 230 will be almost automatic, provided sufficient heat is supplied.

After all of the lithium hydroxide has been added to complete the neutralization of the dicarboxylic acid the temperature of the grease mixture is raised to bring about dehydration. Usually this will take place at about 280 to 300 F. Following dehydration of the grease mixture, it is preferred to raise the temperature further to about 380 to 400 F. and to maintain it at that level for about 15 minutes to 1 hour so as to ensure optimum soap dispersion and improved yields. This increase in temperature to 380 to 400 F. is brought about as rapidly as possible so as to save time as well as to minimize oxidation. The soap stock is then cooled as rapidly as possible, and this cooling is aided by incorporating the remaining quantity of lubricating oil into the mixture. Mixing can be continued until the grease reaches ambient temperatures. When the temperature has been lowered to about 150 F. other grease additives, if any, that are desired in the grease can be introduced. If the grease is then run through a conventional grease mill the grease can be improved somewhat in yield and in appearance. Suitable grease mills include the Morehouse mill, the Charlotte mill and the Gaulin homogenizer.

Although in most instances the lithium hydroxide that is used in forming either the hydroxy fatty acid soap or the soap of the dicarboxylic acid is most conveniently introduced into the grease kettle as a saturated aqueous solution, it is also possible to use dry lithium hydroxide. Ordinarily, however, the average grease making plant will not have proper facilities for convenient handling of the lithium hydroxide in that form.

Normally a slight excess of lithium hydroxide is used during each of the soap formation stages, above the amount theoretically required for complete neutralization of the acids, this excess being in the range of about 0.2 to 0.4 wt. percent of free alkali expressed as sodium hydroxide (ASTM D-128). This is not a critical aspect of the invention, however, as it is also possible to make a high dropping point grease by the process of this invention wherein the soaps can be very slightly on the acid side.

It is to be noted that the formation of the hydroxy fatty acid soap should be undertaken before the dicarboxylic acid soap is prepared. If the steps are reversed, an undesirable grease will result.

The nature of this invention and the manner in which it can be practiced will be better understood by the following examples, which include a preferred embodiment.

EXAMPLE 1 A grease was prepared in accordance with this invention in the following manner. A grease kettle was charged with 333 grams of a base oil identified as LCT-20 base, which was a solvent refined and hydrofinished naphthenic distillate having a viscosity of 315 SUS at 100 F. and a VI. of 67. Then grams (0.233 mole) of 12-hydroxy stearic acid was dissolved in the oil with mild heating, followed by slow addition of a concentrated aqueous solution containing 0.24 mole of lithium hydroxide while the temperature was raised to 230 F. After the addition of the lithium hydroxide, the temperature was quickly raised to 300 F. to dehydrate the soap stock. Then the mixture was quickly cooled to about 205 F. after which 24.13 grams (0.128 mole) of azelaic acid was added. Following stirring of the mixture for about 10 minutes an aqueous solution containing 0.28 mole of lithium hydroxide was added slowly to saponify the azelaic acid while the temperature was being raised to 230 F. The addition of the lithium hydroxide took about 45 minutes.

When all of the lithium hydroxide had been added the temperature was raised to 300 F. and held there for one hour to complete the dehydration. The temperature was then rapidly increased to 390 P. where it was held for about 20 minutes. Thereafter the soap stock was cooled rapidly while an additional 184 grams of the LCT-20 base oil and 316 grams of a parafl'inic oil were added, these added portions of oil being at ambient temperature and thus contributing to the rapid cooling of the mixture. The paraffinic oil was derived from a paratfinic distillate by phenol extraction and solvent dewaxing to +30 F. pour point and had a viscosity index of 75 and a viscosity of 155 SUS at 210 F. Stirring was continued until the grease had cooled to about 150 F. at which time it was milled in a conventional grease mill and cooled to room temperature. The final grease had a dropping point of 625 F. and a penetration of 266 ASTM units at 77 F. The yield of grease was 10.1%. As is well understood in the grease art, the total weight of acids used, expressed as a percentage of the total weight of finished grease, is referred to as the yield.

COMPARATIVE EXAMPLE Weight Component Grams percent 12-hydroxy stearic acid 190. 5 17. 85 Azelaic acid 66. 2 6. 21 LiOH-HzO-- 59. 75 5. 60 Base oil 750.0 70. 34

The preparation in accordance with the present invention was as follows. The base oil was charged to the grease kettle and the l2-hydroxy stearic acid was added to the oil after it had been heated to about 180 F. The base oil was LCT-20 base, which is described in Example 1. The 12-hydroxy stearic acid was then neutralized with a hot saturated aqueous solution of 28.25 grams of lithium hydroxide at about 180 to 200 F. and the mixture was then dehydrated in a manner similar to that of Example 1, i.e., with heating to 300 F. The mixture was then cooled to about 230 F. and the azelaic acid was added and stirred in for a few minutes. Then the second portion of lithium hydroxide (i.e., 31.5 grams) was added as a hot aqueous saturated solution, the temperature of the mixture being maintained at about 215 to 230 F. The grease was then dehydrated by heating to 300 F. after which the temperature was raised to 390 F. and held there for about 30 minutes. At this stage a sample was removed for determination of latent heat of fusion by differential thermal analysis, which is a procedure that is well known in the art.

A second grease was prepared in which 12-hydroxy stearic acid was neutralized with lithium hydroxide at 180 to 200 F. in the same manner as described above, but the resulting mixture of lithium hydroxy stearate and oil was not subjected to the dehydration step. Instead the mixture was heated to about 210 F., the azelaic acid was added and stirred in for minutes, and the second portion of lithium hydroxide (31.5 grams) was added as a hot aqueous saturated solution. After all of the lithium hydroxide had been added, the temperature of the grease mixture was raised to 300 F. to bring about substantially comvplete dehydration after which the mixture was heated to 390 F. and held there for 30 minutes as in the abovedescribed preparation. At this stage a sample was taken for measurement of latent heat of fusion by differential thermal analysis.

The two greases prepared as described were subjected to the latent heat of fusion measurement and their dropping points were also determined. The results obtained are shown below.

Latent heat of Dropfusion, ing cal./g. point, F.

Grease of invention 3. 16 605+ Comparative grease A 6. 79 398 Weight Component Grams percent 12-hydroxy stearic acid 190. 5 19. 68 LlOH-H2O 28. 25 2. 92 Base oil 750. 0 77. 4

The procedure used in making this grease was essentially the same as the first step in the above-described comparative preparations, i.e., the 12-hydroxy stearic acid was added to the hot base oil at about F. and neutralized at about 180 to 200 F. with a hot saturated aqueous solution of the lithium hydroxide. After the grease was dehydrated by heating to 300 F. a sample was removed for dilferential thermal analysis. The latent heat of fusion for grease B was 7.83 calories per gram.

The above comparative examples show that in the case of the grease made in accordance with the present invention there has been an interaction between the lithium 12-hydroxy stearate and the dilithium azelate, there being an indication that the dilithium azelate has become incorporated into the crystal lattice of the lithium 12-hydroxy stearate, thus altering its structure and lowering the latent heat of fusion. In the case of the lithium hydroxy stearatedilithium azelate grease (comparative grease A) made not in accordance with the present invention, the latent heat of fusion was 6.79 calories per gram, this being only slightly less than in the case of comparative grease B, which contained only lithium hydroxy stearate thickener. It is thus evident that here the dilithium azelate has not affected the crystal lattice of the lithium hydroxy stearate. It is to be noted that dilithium azelate itself does not melt until well above 500 F; thus the latent heat of fusion of dilithium azelate does not enter into the picture.

The grease compositions of this invention can also contain various other additives, as is understood by those skilled in this art. Such additives include, but are not limited to, dyes, antioxidants such as phenyl-alpha-naphthylamine, rust inhibitors such as barium dinonyl naphthalene sulfonate, odor modifiers, tackiness agents, extreme pressure agents, and the like.

This invention is not to be limited to the specific examples given herein by way of illustration. Its scope is de fined by the appended claims.

What is claimed is:

1. A process for preparing a lubricating grease of high dropping point, said grease comprising a major proportion of a lubricating oil and a grease thickening proportion of a combination of:

(a) lithium soap of a C to C hydroxy fatty acid and 7 (b) lithium soap of C to C aliphatic dicarboxylic acid,

which comprises the steps of adding a C to C aliphatic dicarboxylic acid to a mixture of lubricating oil and lithium soap of C to C hydroxy fatty acid wherein the mole ratio of said soaps is within the range of about 0.2 to about 1 mole of dicarboxylic acid per mole of hydroxy fatty acid, and then converting the said dicarboxylic acid to its dilithium soap at a temperature within the range of about 190 F. and 240 F., whereby a-complex between (a) and (b) is formed.

2. Process as defined by claim 1 which includes the steps of forming said lithium soap of C to C hydroxy fatty acid in situ in lubricating oil, thereafter substantially dehydrating the resulting soap composition, adding said dicarboxylic acid to the substantially dehydrated soap composition, and then adding to said mixture suflicient lithium base to convert the dicarboxylic acid to its dilithium soap while heating the said mixture within the range of about 190 to 240 F.

3. Process as defined by claim 1 wherein the formation of said complex is promoted by the stepwise addition of lithium base to the mixture of lubricating oil, dicarboxylic acid and lithium soap of hydroxy fatty acid, whereby there is initially present less than sufiicient lithium base to convert the dicarboxylic acid to its dilithium soap.

4. Process as defined by claim 1 which includes the steps of dehydrating the composition subsequent to the formation of said dilithium soap and rapidly increasing the temperature to a range of about 380 to 400 F. for a brief period to thereby improve soap dispersion, and then rapidly cooling the composition.

5. Process as defined by claim 1 wherein the lithium soap of hydroxy fatty acid is that of a C to C hydroxy fatty acid.

6. Process as defined by claim 1 wherein the dicarboxylic acid is a C to C dicarboxylic acid.

7. Process as defined by claim 1 wherein said hydroxy fatty acid is l2-hydroxy stearic acid.

8. Process as defined by claim 1 wherein said dicarboxylic acid is azelaic acid.

9. A lubricating grease prepared by the process of claim 1, wherein the mole ratio of dicarboxylic acid to hydroxy fatty acid is within the range of about 0.2 to 1 to about 0.8 to 1.

10. A lubricating grease as defined by claim 9 wherein said lithium soaps are soaps of l2-hydroxy stearic acid and of azelaic acid.

References Cited UNITED STATES PATENTS 2,898,296 8/1959 Pattenden et a1. 2524l 2,940,930 6/1960 Pattenden et a1. 25241 2,846,392 8/ 1958 Morway et al 252-41 3,681,242 8/1972 Gilani et al. 252-41 DANIEL E. WYMAN, Primary Examiner I. VAUGHN, Assistant Examiner 

